1. Field of the Invention
The present invention relates to a novel lighting circuit for a vehicular discharge lamp. More specifically, this invention pertains to a novel lighting circuit for a vehicular discharge lamp which ensures protection against a variation in DC input voltage, as well as temporarily inhibits power supply to the discharge lamp in response to a temporary change in DC input voltage, and restarts power supply to the discharge lamp when the DC input voltage returns within a predetermined range.
2. Description of the Related Art
Recently, some circuit protection measure is taken against a variation in voltage of a battery serving as a power supply for lighting a compact metal halide lamp which has been receiving attention as a light source for automobiles.
This protection measure is taken to prevent the unstable lighting of the lamp due to a change in battery voltage and malfunction of the lighting circuit. As an example of the protection means, a battery voltage detector and a power cutoff circuit (comprising a relay, etc.) are provided so that power supply to the lamp is stopped when the voltage variation does not fall within a range which ensure the normal operation.
The inhibition of power by the power cutoff circuit continues unless a lighting switch is set on again.
According to the conventional lighting circuit, the above protection measure functions as expected when a continuous abnormal status of the battery voltage happens. However, the protection measure also cuts off power supply to the lamp in response to a temporary change in battery voltage. When the battery voltage immediately returns to the allowable range, the lamp will be kept off.
From a viewpoint of safe driving at night, the lamp being switched off for each temporary change in battery voltage and this OFF status being kept until the lighting switch is set on again means that a driver should drive a car in the dark at his or her own's risk. In other words, it is desirable that the lamp be lit again as much as possible when a change in battery voltage is temporary and does not influence at all on relighting the lamp.
FIG. 8 is a diagram for explaining this situation. In the diagram, "V.sub.B " denotes the battery voltage, ".DELTA.V.sub.L " a variation range for the lower limit of the battery voltage, and ".DELTA.V.sub.H " a variation range for the upper limit thereof. "V.sub.L " indicates the center level of .DELTA.V.sub.L while "V.sub.H " indicates the center level of .DELTA.V.sub.H. These variations would occur as variation in produced lamps and lighting devices.
In view of the variations, it is necessary to set the lower and upper limits (V*.sub.L and V*.sub.H, respectively) of the allowable range of the battery voltage V.sub.B as follows: EQU V*.sub.L =V.sub.L +.DELTA.V.sub.L /2 EQU V*.sub.H =V.sub.H -.DELTA.V.sub.H /2
The allowable range .DELTA.V.sub.B for V.sub.B will therefore be: EQU .DELTA.V.sub.B =V.sub.H -V.sub.L -(.DELTA.V.sub.L +.DELTA.V.sub.H)/2
The time chart illustrated below the range in FIG. 8 shows the operational status of the power cutoff circuit: "OFF" indicates the disabled state of the power cutoff circuit or power supply to the lamp being allowed, and "ON" represents the power supply to the lamp being inhibited by activation of the power cutoff circuit.
As illustrated, when the battery voltage V.sub.B gradually falls from within the range of V*.sub.L .ltoreq.V.sub.B .ltoreq.V*.sub.H down to V.sub.B =V*.sub.L at point P, power supply to the lamp is stopped. The power supply to the lamp will not be done even when V.sub.B returns later to the range of V*.sub.L .ltoreq.V.sub.B .ltoreq.V*.sub.H, passing point Q (V.sub.B =V*.sub.L).